Spatially variable slit for mass spectormeter apparatus



Sept. 28, 1965 w. M. HICKAM ETAL SPATIALLY "ARIABLE SLIT FOR MASS SPECTROMETER APPARATUS 7 Sheets-Sheet 1 Filed Nov. 6, 1961 William M. H'

George G. Sweeney. *jM M M ATTORNEY Sept. 28, 1965 w. M. HICKAM ETAL SPATIALLY VARIABLE SLIT FOR MASS SPEGTROMETER APPARATUS Filed Nov. 6, 1961 '7 Sheets-Sheet 2 Fig. 4.

SPATIALLY VARIABLE SLI'I FOR MASS SPECTROMETER APPARATUS '7 Sheets-Sheet 3 Filed Nov. 6, 1961 mMhMEOKFUMJm OF p 23, 1965 w. M. HICKAM ETAL 3,209,143

SPATIALLY VARIABLE SLIT FOR MASS SPECTROMETER APPARATUS Filed Nov. 6. 1961 7 Sheets-Shee+ 4 Se t. 28, 1965 w. M. HICKAM ETAL SPATIALLY VARIABLE SLIT FOR MASS SPECTRQMETER APPARATUS '2' SheetsSheet 5 Filed NOV. 6. 1961 mokum imm 20H Sept. 28, 1965 w. M. HICKAM ETAL SPATIALLY VARIABLE SLIT FOR MASS SPECTROMETER APPARATUS Filed Nov. 6, 1961 7 Sheets-Sheet 6 Fig.9.

p 23, 1965 w. M. HICKAM ETAL 3,209,143

SPATIALLY VARIABLE SLIT FOR MASS SPECTROMETER APPARATUS Filed Nov. 6, 1961 7 Sheets-Sheet 7 United States Patent ice 3,209,143 SPATIALLY VARIABLE SLIT FOR MASS SPECTROMETER APPARATUS William M. Hickam, Churchill Borough, and George G.

Sweeney, Penn Hills, Pa., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Filed Nov. 6, 1961, Ser. No. 150,447 11 Claims. (Cl. 250-413) This invention relates to the electric discharge art and has particular relationship to mass spectrometers and generally related apparatus for depositing materials 1n dependence upon the masses of their components. ThlS invention in its broader aspects relates to apparatus for depositing isotopes of a polyisotopic material.

In mass spectrometers a beam of positive or negative ions which may be derived from a spark source, is projected on a monitor electrode or slit. The resulting beam emitted by the monitor slit is deflected by a magnetic field and onto a collecting or indicating medium such as a photographic plate. The extent of the deflection in the beam produced by the magnetic field is dependent on the ratio of the mass to the charge of the ions in the beam, ions of different mass-to-charge ratios being deflected differently. Where the charges are alike as is predominant, the deflection depends only on the mass. The ions 1mpinging on the photographic plate produce a line or lines corresponding to the mass content of the beam. The lines produced by the heavier ions are more remote from the monitor slit than are the lines produced by the lighter One of the important uses of mass spectrometers of the type under consideration is to determine the concentration of diflerent mass components of material which is being analyzed. This material may be included in one or both of the electrodes between which the spark is produced. The concentration of the diiferent components is determined by comparing the intensities of the lines corresponding to the masses of the components produced on the photographic plate. In a typical situation it is necessary that the concentration be determinable over a range of at least one million. The intensity variation available must then extend over a range of at least one million.

The reasonably measurable intensity variation for a photographic plate at any exposure is only of the order of three. To attain the one million intensity variation in accordance with the teachings of the prior art, it is necessary that a large number of different exposures be made for each sample. This is highly time consuming.

It is then an object of this invention to provide a mass spectrometer in the operation of which it shall be feasible to analyze a sample for the concentration of mass components over a range of at least one million in a relatively short time interval. It is an incidental object of this invention to provide a monitor electrode or slit for such a mass spectrometer. A more general object of this invention is to provide electromagnetic apparatus for producing a deposit on a collector varying in thickness or intensity over a predetermined range.

In accordance with this invention in its specific aspects, a mass spectrometer is provided which includes a monitor slit, the beam transmission area of which throughout the cross section of the transmitted beam is variable over a substantial range. Typically, the slit may be of the stepped type having for example, five steps, each step having the same height but a width of three times the next narrowest step. Under such circumstances, the intensity of the portion of the beam transmitted by the widest step is approximately 81 times the intensity of the portion of the beam transmitted by the narrowest step. Concentra- 3,209,143 Patented Sept. 28, 1965 tion may be measured over a range of one million with a mass spectrometer having such a Monitor Electrode with only three exposures in a short time.

The novel features considered characteristic of this invention are disclosed generally above. For a more thorough understanding of this invention both as to its organization and as to its method of operation, together with additional objects and advantages thereof reference is made to the following description taken in connection with the accompanying drawings, in which:

FIGURE 1 is a view in perspective showing a mass spectrometer in accordance with this invention;

FIGS. 2A, 2B, and 2C together are a view in longitudinal section of the spectrometer shown in FIG. 1;

FIG. 3 is a fragmental view of a portion of the spectrometer shown in FIG. 1;

FIG. 4 is a view in section taken along line IV-IV of FIG. 3;

FIG. 5 is a view in section taken along line VV of FIG. 2;

FIG. 6 is a view in perspective with the cover and another part removed of the deflection chamber of the spectrometer shown in FIG. 1;

FIG. 7 is a view in front elevation of a monitor electrode or slit used in a spectrometer in accordance with this invention in one of its specific aspects;

FIGS. 8 and 9 are slits of spectrometers in accordance with a modification of this invention;

FIG. 10 is a view in side elevation of a monitor electrode in the form of a screen of a spectrometer in accordance with a further modification of this invention; and

FIG. 11 is a copy of a photograph having three exposures produced in the practice of this invention.

The apparatus shown in the drawings is a mass spectrometer including a Beam Generator, a Velocity Selector, a Monitor Electrode, an Ion Deflector and a Photographic Plate Assembly. Ions from the spark S between the electrodes E1 and E2 of a material under test are accelerated in the Beam Generator to form a beam. The ions in the beam may be either positive or negative depending on the material under analysis. The beam is transmitted through the Velocity Selector which operates electrostatically to spread the beam in accordance with the velocities of the ions. The Velocity Selector may be set to select a predetermined range of ion velocities for analysis. The ions selected are concentrated in a subbeam which is transmitted through the Monitor Electrode which in the practice of this invention is a slit or screen of progressively varying area. The beam transmitted by the Monitor Electrode is deflected magnetically by the Ion Deflector onto the photographic plate of the Photographic Plate Assembly. Ions of different masses impinge on dif ferent parts of the plate in dependence upon their masses.

The ion beam is produced in a highly evacuated chamber or housing of a material such as stainless steel in cluding a plurality of wall sections each joined to the end of another. These include a generally cylindrical section 21 in which the Beam Generator is enclosed, and a section 23 terminating in a part 25 extending at an obtuse angle to the section 21. The section 23 houses the Velocity Selector and the Monitor Electrode. To the section 23 a section 27 is connected; this extends largely at an obtuse angle to the part 25 and houses the Ion Deflector and Photographic Plate Assembly. The cylindrical section 21 is closed at its end by a plate 29 which carries the spark electrode assembly. The section 23 is sealed vacuum-tight to the section 21, the section 27 to the section 23 and the plate 29 to the end of the section 21. To assure a vacuum-tight enclosure a liquid Teflon seal is provided at these joints. The section 27 is closed at the end by a cross-wall section 31 which has a slot 33 for inserting the photographic plate 35 and its holder. A plate 37 is mounted on the wall 31 and carries a plug 39 for sealing the slot 33.

The section 21 has an exhaust opening 41 at the end remote from the plate 29 and the section 23 has an exhaust opening 43 at the end near section 21. The section 27 has an opening 45 near the section 31. This opening 45 may serve for connecting an electrometer (not shown) or for exhaust. The cylinder 21 also includes a window 46 through which the spark 7 may be observed.

The Beam Generator includes the spark electrodes E1 and E2 which are adjustably mounted in tubes 51 extending at an angle to the plate 29 and welded to this plate. Each tube 51 has secured thereto a generally elongated cup-shaped extension 53 having a shouldered opening 55 in its cap 57. Each electrode E1 and E2 is held in a pinvice 61 screwed onto a stem 63 which extends into an insulating glass bead 65. Another rod or stem 67 extends from the bead 65 through the end 69 of the tube 51 into the tube or sleeve 53. A Sylphon or bellows 71 is mounted on the end 69. This Sylphon 71 has a movable head 73 from which inner and outer stems 75 and 77 extend. The rod 67 is secured in the inner stem 75. A threaded rod 79 is screwed into the stem 77 and locked by a nut 81. The rod 79 passes through a swivel joint 83 in the cap 57 and sleeve 85 engaging the joint and is rotatable by a thumb-nut 87 engaging the sleeve 85 into which it is screwed.

The swivel joint 83 includes a two-part block 91 within which a ball 93 is enclosed. The ball 93 is movable within the block 91. The block 91 is held slidably within the cap 57 by a cover 95. The cap 57 has projections 97 and 99 at right angles each enclosing a spring 101 within a sleeve 103. Screws 105 and 107 opposite to the projections screw into the cap 57 and serve to set the position of the block 91.

The end plate or header 29 includes tubes 111 carrying seals 113 through which the hollow conductors 115 which carry the high frequency power to the spark S are sealed. The spark potential may have a frequency of the order of 800 kilocycles and a potential of 100 kilovolts.

The Beam Generator includes a conducting ring 121. This ring 121 is supported by L-shaped brackets 123 and 125 supported internally by the header 29 on insulating supports 127. Direct-current beam power of between 5,000 and 20,000 volts (typically about 12,000) is supplied through a conductor 131 which insulatingly passes through one of the tubular conductors 115 and is connected to one of the pin-vices 61. This pin-vice is also connected to the ring 121. The other terminal of the beam-power supply is grounded as is the housing. The ring 121 accelerates the ions in the spark S. The beam potential is positive if the ions are positive and negative if the ions are negative.

The Beam Generator also includes an accelerating electrode assembly 135 which extends from the outwardly projecting stem 137 of an inner header 139. The accelerating assembly 135 includes a sleeve 141 secured to the stem 157. A ring 143 is secured to the end of the sleeve 141 and a cup-shaped cap 145 having a small central opening 147 is secured to this ring 143. The ring 143 also carries a plate 148 having an opening 149 co axial with the opening 147. Another plate 151 having an opening 153 coaxial with the openings 147 and 149 is carried by the header 139 at the base of the stem 137. The openings 147, 149, 153 cooperate to collimate the ion beam. A protective screen 157 is suspended from the inner header 139.

The inner header 139 includes an outwardly extending stem 161 from which the Velocity Selector is suspended. The Velocity Selector includes an electrostatic field plate assembly 163 and a slit 165 for selecting the velocity range of the ion beam to be transmitted. The electrostatic field plate assembly 163 is curved and extends into the portion 25 of the section 23 which is bent at an angle.

The field plate assembly includes curved supporting plates 171 suspended from opposite fiat areas on the stem 161. Curved electrostatic plates 173 are secured to brackets 175 mounted on insulating (glass) blocks 177 between the plates 171. The surfaces of the plates 173 are at right angles to the surfaces of the plates 171. About 600 to 800 volts are impressed between the plates 173. The connection is effected through two of the three port holes 181 in the square portion 153 of section 21 (FIG. 1).

The Velocity Selector also includes a disc 191 having a hole in its center. This disc 181 is mounted in a shouldered block 193 at the end of the supporting plates 171. The hole in disc 191 determines the ion velocity region of the beam.

The Monitor Electrode may have the form illustrated in FIG. 7, 8 or 9. This electrode is a circular plate 201 having a symmetrical opening 203 of variable area throughout the cross section of the beam. The Monitor Electrode is secured between a ring 205 of Pyrex glass mounted coaxially with the hole in the Velocity Selector disc 191 and a shouldered Pyrex glass block 207 which is secured to the block 193 holding the Velocity Selector disc 191. The Monitor Electrode is properly set so that the portion of the beam transmitted by the Velocity Selector plate extends uniformly over the slit or opening 203 in the Monitor Electrode. The Monitor Electrode is connected to an electrometer or other instrument so that the current or charge of the ion beam can be measured. This measurement can be used to monitor exposure of the photographic plate 35 in lieu of timing. The cross sectional area of the beam transmitted by the different parts of the opening in the Monitor Electrode is then determined by the respective areas of these openings. The ion current transmitted by these respective areas then varies in accordance with the area and the intensity of the indication produced on the photographic plate 35 varies correspondingly.

The Monitor Electrode shown in FIG. 7 is made up of two semi-circular halves 211 and 213 of a hard material such as tantalum. A stepped groove 215 is precisely machined in each half. The heights h of the steps of each groove 215 are equal and the boundaries of the successive steps are spaced from the edge of the semi-circular disk distances d which increase in geometric progression. In a typical situation the progression factor may be three. After the grooves are precisely machined in the semicircular halves 211 and 213, the halves are mounted with their diameters abutting and with the grooves coextensive to form a stepped slit or slot 219. The two halves 211 and 213 are then joined into a single circular unit by platinum strips 221 that are welded to the two halves.

The grinding of the steps should be highly precise. The width of the steps in the slit should be maintained to within, at worst, plus or minus .0002 inch. The steps of the grooves instead of being ground mechanically may be ground by electron beam cutting or by etching techniques.

The steps of the slit 219 are of areas increasing in geometric progression from a small to a large area. The heights h of the steps and the width of the step of largest area are such that the slit is covered by the beam of cross-section passing through the opening in the Velocity Selector disc 191. The cross section of the beam is shown in dotted lines in FIG. 7. The Monitor Electrode E shown in FIG. 8 is similar to the electrode shown in FIG. 9 and is made similarly except that the slit 231 formed has a curved boundary instead of a stepped boundary. The groove forming the halves of the slit shown in FIG. 8 could be curves defined by an equation such as:

in which X is the distance of any point on the curve from the diametrical edge of the semi-circular half and y is the height of the point under consideration, and a and b are constants.

FIG. 9 is'a like Monitor Electrode in which the slot openings 233 and 235 are separated. This slit may be produced by grinding in each of the halves an opening bounded by a curve on one side and by a right angle on the other side.

A slit as shown in FIG. 10 may also be used. This is a screen or grid 241 in the form of a geometric surface, for example, a surface of revolution. This grid 241 is mounted convexed or concaved towards the Velocity Selector. The curvature of the grid 241 should vary sharply throughout the cross section of the beam. Since the beam area transmitted by the grid varies with the angle of the grid 241 to the beam, the maximum transmission is at the center of the grid and the minimum at the region of intersection of the rim of the beam with the grid.

The housing 27 for the Ion Deflector is a generally rectangular box. (See FIG. 6.) This housing is made up of a box 251 having open opposite sides secured and sealed to flanges 253 from the portion of the section 23 and of box-like covers 255 and 256 which extend into and are welded in the openings. The covers 255 and 256 constitute the poles of an electromagnet 257. The magnet supplies a field of about 12,000 gauss. The ends of the pole faces 255 and 256 adjacent the Monitor Electrode extend between the boundary 257 of the lowest area step of the Monitor Electrode and the boundary 259 of the greatest area step and are very near to the boundaries as shown in broken lines in FIG. 7. The cover 256 has a groove providing a space 261 within which the Photographic Plate Assembly is mounted between the cover 256 and one of the adjacent walls of box 251. The groove 261 is coextensive with a slot 262 in the plug 39 into which the photographic plate extends.

The Ion-Deflector housing also includes a plate 271 of transverse (perpendicular to plane of FIG. 2) C section (see FIG. 4) which holds a backing plate 273 of the Photographic Plate Assembly. The plate 271 of C section is connected to a photographic-plate displacing or moving mechanism 281 which is sealed through a wall of the enclosure 251. This moving mechanism includes a rotatable knurled head 283 extending from the wall of the enclosure and a gearing system (not shown) for converting the rotation of the knurled head into a translational motion of the C plate 271 in a direction perpendicular to its long dimension. The position of the plate 271 may be determined with reference to a mark dimension on a bracket 285 extending from the wall of the enclosure 251.

For insertion in the Ion Deflector, the photographic plate 35 is mounted on the backing plate 273 and is held in engagement with this plate by a generally C section strip 291 (FIG. 4). The assembly of the C section strip 291, the backing plate 273 and the photographic plate 35 are slid into the C plate 271 movable by the knurled disk 283 through the oval-shaped opening 293 in the plate 37 and the slot 33 in the end 31 of the enclosure section 271 to a position to receive the deflected ion beam. This opening is closed by the oval plug 39. The plug 39 may be locked against O-rings 295 by a clamp 297 which may be slipped into a slotted fixture 299 and turned to the holding position. A bolt 301 passing through the clamp may then secure the plug 39 against the O-rings 295.

In the use of the apparatus, the spark electrodes E1 and E2, the accelerating electrode assembly 135, the Velocity Selector, and Monitor Electrode are precisely aligned with the apertured plates 147, 149 and .153 coaxial to produce a collimated beam and the opening 165 set to transmit a beam over the desired velocity range. The Monitor Electrode is set so that the beam covers the opening 219 and is perpendicular to it. The Photographic Plate Assembly is mounted in the Ion Deflector in position to receive the impressions from the ion beam.

The knurled head 283 is preferably turned to a position such that the first exposure is along one longitudinal margin of the plate 35. The Velocity Selector voltage impressed between the selector plates is of the order of of the accelerating voltage. The enclosure is then evacuated to a pressure of the order of 1() to 10 millimeters of mercury. After the evacuation is completed, the spark S is fired between the electrodes E1 and E2 and the potentials are impressed on the accelerator, the Velocity Selector and the magnetic field. The plate 35 is then exposed to the beam from the spark for a predetermined time interval or until the Monitor Electrode collects a predetermined charge.

The exposure of the photographic plate 35 may be timed in diflerent ways. For example, the spark S or the Velocity Selector voltage between the electrostatic plates 173 may be maintained during the exposure time or changed after the time. The effect of disconnecting the Velocity Selector voltage or even of changing it is to cause the beam to impinge on the walls of the Velocity 'Selector aperture 191 and the beam then does not reach the photographic plate 35. Once this exposure has timed out, the photographic plate 35 is moved by means of the knurled bead 283 and a second different exposure is produced. On the completion of this exposure, a third exposure may be made.

A typical exposed plate produced in this way is shown in FIG. 11. FIG. 11 was produced with a stepped slit as shown in FIG. 7. This view shows a photographic plate 35 which has received three different exposures and has three records a, b and c as presented on FIG. 11. Record a corresponds to the shortest exposure and record c to the longest exposure. In each record the upper portion corresponds to the smallest step of the Monitor Electrode and the lowest portion to the largest step of the Monitor Electrode. Thus the lower portions of each of the lines 311 reproduced have an intensity of about 81 times the upper portion. In producing FIG. 11, the exposures were set so that the minimum intensity of record b is approximately equal to the maximum intensity of record a and the minimum intensity of record 0 approximately equal to the maximum intensity of record b.

In interpreting the records shown in FIG. 11, it may be assumed that the strongest impression in the upper portion of record a corresponds to a concentration of one of the corresponding element. The weakest impression in the upper portion of record a then corresponds to a concentration of A of the corresponding element and the weakest impression in the lowest portion of record a corresponds to a concentration of The strongest impression in the upper portion of record b then corresponds to a concentration of and the weakest impression in the lowest portion of record b corresponds to a concentration of squared or about Similarly, the weakest impression in the lower portion of record 0 corresponds to a concentration of multiplied by 1 or about 1A,.100000. Thus, with only three exposures a wide range of concentration may be measured. Stated another way record a measures concentration between 1 and record b between and $6 and record 0 between /qg ogo and /1,400,000-

In addition to serving for producing photographic impress-ions of the Ion beams the apparatus in accordance with this invention may be used for depositing variable concentrations of isotopes or other components of different masses.

To aid in the understanding of this invention the following summary is presented:

In the spark-source mass spectrometer the positive (or negative) ion mass spectrum derived from spark of a sample is recorded on a photographic plate. To use the instrument in the analysis of impurities in solids over a concentration range of 1 in it is necessary because of the limited range (about 3) of sensitivity of the photographic plate to record some 12 to different exposures of measured relative intensity. Once a single exposure intensity has been set the useful concentration range for which information can be obtained is limited to the useful range of the photographic plate detector.

In obtaining the relative intensity of the various exposures a Monitor Electrode or screen at the end of the electrostatic analyzer samples a portion of the ion beam. The total charge (ion beam cross section) arriving at this Monitor Electrode is used in obtaining the spectrum of varying intensity. The time to obtain 12 to 15 exposures is of the order of 1 to 1 /2 hours on prior art apparatus. The interpretation placed on the information recorded depends upon the accuracy to which the relative intensity of the various exposures is measured.

This invention relates in particular to the use of a variable-slit Monitor Electrode in this apparatus, or in general to the use of a variable slit for varying the transmitted fraction of a positive or negative ion beam in a defined manner. Specifically, the variable slit may be a stepped slit. In the practice of this invention in its specific aspects as shown in FIGS. 1 and 2 the stepped Monitor Electrode is located after the exit velocity range defining slit of the electrostatic analyzer. The specific electrode or slit shown in FIG. 7 has five defined areas. The ion beam approaching the stepped-slit is of nearly uniform density over an area larger than that associated with the maximum opening in the stepped slit. Each step in the stepped-slit transmits an ion beam linearly proportional to its area. The areas have been chosen to have a ratio to each other of approximately 3 to 1. The five slits then provide five distinct ion beams having relative intensity of 1, 3, 9, 27, 81 approximately. This makes possible the covering in a single exposure a concentration range of 81 for which the photographic plate is sensitive rather than 3. Three exposures using the stepped-slit of FIG. 1 then allow a coverage of concentration of approximately 10 instead of the usual fifteen exposures.

In addition to the advantages gained in establishing a wider concentration for elements under the same source condition, it is expected that this technique could be of great advantage in using the instrument for isotopic analysis. It is particularly advantageous in working with small samples, where because sample size alone only a single exposure is practicable. In such samples (10*- 10" gms.) the stepped-slit technique allows the photographic plate to be sensitive over a wider range of concentration than is practicable otherwise with a single exposure. In segregation studies the wider range could be useful in establishing the concentration levels of the segregated materials.

In the use of the mass spectrometer ion beam for depositing doping films of a specified monoisotopic or singleelement species to a material (for example for solidconductor devices) the stepped-slit could provide a more reproductive and easier means of altering the rate of deposition than either electrostatic or electromagnetic means available in accordance with the teachings of the prior art. In particular, it could readily provide one species at a high concentration and a second at low concentration for which the ratio could be specified or varied without altering the conditions in the ion source.

This invention may also be used to produce mass spectrographs for gases. In this case an electron beam source is provided for ionizing the gas. Similarly, the electron beam could be used to ionize vapors of vaporized solid materials.

While preferred embodiments of this invention have been disclosed herein, it is understood that many modifications thereof are feasible. This invention then is not '8 to be restricted except insofar as is necessitated by the spirit of the prior art.

We claim as our invention:

1. A mass spectrometer including a source of ions, means connected to said source for producing a beam of said ions, means connected to said producing means for deflecting the ions of said beam in dependence upon their mass thus producing a beam of deflected ions, an ion-sensitive surface, means mounting said surface so that said deflected ion beam impinges thereon, and means interposed between said surface and said source for varying the intensity of the deflected ion beam impinging on said surface in geometric progression along one dimension of said surface.

2. An electrode for a mass spectrometer, said electrode consisting of a plurality of component plates each composed of a hard material such as tantalum, each plate having ground therein a precise step-shaped groove, said plates being secured together in edge abutting relationship with said grooves coextensive so that said component plates form a composite plate having therein a stepshaped opening formed by said coextensive grooves.

3. An electrode for a mass spectrometer consisting of a plurality of component plates each composed of a hard material such as tantalum, each plate having ground therein a precise step-shaped groove, said plates being secured together in edge abutting relationship with said grooves coextensive so that said plates form a composite plate having therein a step-shaped opening formed by said grooves, said component plates being held together to form said composite plate by strips of a material such as platinum bridging the junction of said plates and joined to the plates.

4. An electrode for a mass spectrometer consisting of a plurality of component plates of a hard material such as tantalum, secured together in edge abutting relationship to form a composite plate by strips of a material such as platinum extending across the junction of said component plates and joined to said component plates, each component plate having ground therein precisely a groove having the form of the boundary of the portion of a stepped slit, said component plates being secured together with said grooves coextensive to form a stepped slit in said composite plate.

5. A mass spectrometer including a source of ions, means connected to said source for producing a beam of said ions, means connected to said producing means for deflecting the ions of said beam in dependence upon their mass, photographic means in the path of said deflected ions for producing an indication of the relative positions of said ions, slot means permeable to said beam interposed in the path of said beam for transmitting to said photographic means only a portion of said beam, the area per unit length permeable to said beam of said slot means varying from a small magnitude to a relatively substantial magnitude, the said length along which the permeable area varies as aforesaid being along a dimension of said slot means corresponding to the one of the dimensions of said photographic means which one dimension is generally transverse to the direction of deflection along said photographic means.

6. A mass spectrometer including a source of ions, means connected to said source for producing a beam of said ions, means connected to said producing means for deflecting the ions of said beam in dependence upon their mass, collecting means in the path of said deflected ions for collecting said ions, slot means interposed in the path of said beam permeable to said beam for transmitting to said collecting means only a portion of said beam, the area per unit length permeable to said beam of said slot means varying from a small magnitude to a relatively substantial magnitude to vary the relative rate of deposit of said ions on said collecting means in dependence upon the variation in said area per unit length, the said length along which the permeable area varies as aforesaid being along a dimension of said slot means corresponding to the one of the dimensions of said collecting means which one dimension is generally transverse to the direction of deflection along said collecting means.

7. A mass spectrometer including a source of ions, means connected to said source for producing a beam of said ions, means connected to said producing means for deflecting the ions of said beam in dependence upon their mass thus producing a beam of deflected ions, an ionsensitive surface, means mounting said surface so that said deflected ion beam impinges thereon, said ion beam impinging at each instant over an area of intersection of said beam and surface, said area-of-intersection for ions of different masses being at different positions along one dimension of said surface, and ion-beam restricting means interposed between said surface and said source for defining said area-of-intersection so that the intensity of the deflected ion beam in said area-of-intersection along another dimension =of said surface generally transverse to said one dimension varies between a higher magnitude and a lower magnitude.

8. A mass spectrometer including a source of ions, means connected to said source for producing a beam of said ions, means connected to said producing means for deflecting the ions of said beam in dependence upon their mass, indicating means in the path of said deflected ions for producing an indication of the relative positions of said ions, slot means interposed in the path of said beam and having a portion on which said beam impinges and which is permeable to said beam and through which only a portion of said beam is transmitted to said indicating means, the impinging portion of said beam being of substantially uniform density throughout said portion of said slot and said permeable portion varying in area per unit length along a dimension of said slot corresponding to the one of the dimensions of said indicating means generally transverse to the direction of deflection along said indicating means from a small magnitude to a relatively substantial magnitude.

9. A mass spectrometer including a source of ions, means connected to said source for producing a beam of said ions, means connected to said producing means for deflecting the ions of said beam in dependence upon their mass thus producing a beam of deflected ions, an ion-sensitive surface, means mounting said surface so that said deflected ion-beam impinges thereon, and beam-permeable means interposed between said surface and said source, said beam permeable means having a portion on which said beam impinges and which is permeable to said beam and over which portion of said beam-permeable means the impinging portion of said beam is of substan tially uniform density, said beam-permeable portion being dimensioned in a direction along one dimension of said surface to vary spatially along said one dimension of said surface the intensity of the deflected ion beam at any instant incident on said surface so that at any instant said intensity is different at different points along said one dimension.

It). A mass spectrometer including a source of ions, means connected to said source for producing a beam of said ions, means connected to said producing means for deflecting the ions of said beam in dependence upon their mass thus producing a beam of deflected ions, an ionsensitive surface, means mounting said surface so that said deflected ion-beam impinges thereon producing at each instant on said surface an ion-impinged area, and means interposed between said surface and said source for spatially, along one dimension of said area, varying the intensity of the ions incident at dilferent points along said one dimension to produce on said area at any instant a predetermined non-uniform distribution of ion intensity.

11. A mass spectrometer including a source of ions, means connected to said source for producing a collimated beam of said ions, a surface interposed in said beam so that said ions impinge thereon, and a screen having an array of ion-beam impermeable partitions, openings bounded between said impermeable partitions which openings are permeable to said ion beam, said screen being interposed between said source and said collirnating means and said surface and being permeated by said collimated beam, the angle of said openings to said beam varying progressively along one dimension of the cross section of said beam from to a magnitude differing substantially from 90 so that the areas of the openings in said screen multiplied by the sines of the angles respectively of each area to the beam vary progressively along one dimension of the cross section of said beam, to vary correspondingly along said one dimension of said surface at any instant the intensity of the respective portions of said beam permeating said openings and impinging on said surface.

References Cited by the Examiner UNITED STATES PATENTS 2,410,550 11/46 Padva 250237 2,570,124 10/51 Hernqvist 25041.9 2,659,821 11/53 Hipple 25041.9 2,674,698 4/54 Danforth et a1 25041.9 2,851,608 9/58 Robinson 2504l.9 2,852,684 9/58 Payne 25041.9 2,925,496 2/ 60 Zoubek 250 2,961,538 11/60 Bishop 25041.9

RALPH G. NILSON, Primary Examiner. 

1. A MASS SPECTROMETER INCLUDING A SOURCE OF IONS, MEANS CONNECTED TO SAID SOURCE FOR PRODUCING A BEAM OF SAID IONS, MEANS CONNECTED TO SAID PRODUCING MEANS FOR DEFLECTING THE IONS OF SAID BEAM IN DEPENDENCE UPON THEIR MASS THUS PRODUCTION A BEAM OF DEFLECTED IONS, AN ION-SENSITIVE SURFACE, MEANS MOUNTING SAID SURFACE SO THAT SAID DEFLECTED ION BEAM IMPINGES THEREON, AND MEANS INTERPOSED BETWEEN SAID SURFACE AND SAID SOURCE FOR VARYING THE INTENSITY OF THE DEFLECTED ION BEAM IMPINGING ON SAID SURFACE IN GEOMETRIC PROGRESSION ALONG ONE DIMENSION OF SAID SURFACE. 